1. Field of the Invention
This invention relates to a control system for internal combustion engines, and more particularly to a control system of this kind, which controls the engaging force of a lock-up mechanism of an automatic transmission connected to the engine during gear shifting of the transmission.
2. Prior Art
Conventionally, in an automotive vehicle having an automatic transmission installed therein, a shock is generated during gear shifting due to a difference in rotational speed before and after the gear shifting, which incurs uncomfortableness to the driver. Generally, when an automatic transmission is upshifted, the deceleration of the vehicle once increases to decelerate the vehicle upon issuance of an upshifting command, and then a shock is generated due to the acceleration to accelerate the vehicle. To prevent such a shock, a control system for internal combustion engines for vehicles has been proposed, for example, by Japanese Laid-Open Patent Publication (Kokai) No. 5-321707, which increases the output torque of the engine by controlling a throttle actuator of the engine coupled to a throttle valve when an upshift of an automatic transmission of the engine is carried out, and then reduces the output torque below a value before the upshift, to thereby suppress a change in the acceleration acting to accelerate the vehicle during the upshift and a change in the deceleration to decelerate the vehicle, which precedes the change in the acceleration.
Further, a control system for automatic transmissions of automotive vehicles is known from Japanese Laid-Open Patent Publication (Kokai) No. 2-45628, which increases the output torque of the engine when a downshift of the automatic transmission is carried out, to thereby reduce a shock caused by the downshift.
According to the former prior art technique, however, when the output torque of the engine is increased by the throttle valve of the engine, the torque conversion rate decreases due to a characteristic of a torque converter of the automatic transmission, and hence the rotational speed of the engine increases (blows up), which incurs degraded driving comfortableness to the driver, increased fuel consumption, and increased noise and vibrations.
Further, according to the latter prior art technique which only increases the output torque of the engine during a downshift, upon completion of the downshift, the rotational speed of the engine sharply decreases below a lower limit value above which fuel cut is carried out, which causes starting of fuel supply to the engine, resulting in increased fuel consumption. Moreover, this technique still remains to be improved to effectively carry out engine brake, owing to a slip in the torque converter.